For the filtering of various kinds of liquids, the liquid is passed upwardly through a filter housing containing a flat, horizontal, microporous, hydrophilic membrane sealed to the walls of the filter housing and backed up by a flat grid or the like support. It is well known that when a hydrophilic membrane is wet (i.e when the pores are filled with the liquid being filtered), air or other gas cannot pass through the membrane except under increased pressure. Hence, if there is air or other gas in the liquid being filtered, it accumulates on the inlet surface of the membrane and blocks or interferes with the passage of liquid through the membrane. Further, once the membrane is wet, if there is air in the side of the housing underneath the membrane prior to commencing the filtration (as where the filter is periodically used at frequent intervals), the air can't escape except under a high pressure sufficient to drive it through the wet membrane, with threat of membrane rupture. It is well known to form or treat a portion of the membrane to render it hydrophobic so that the air or other gas can exit through such portion. For example, it is known to use a membrane with a hydrophilic central portion and a hydrophobic peripheral portion, or vice versa. But no matter the location of the hydrophobic portion or portions, the possibility remains that air or other gas can accumulate on the hydrophilic membrane portions. This possibility can be diminished by increasing the aggregate area of the hydrophobic portion or portions; however, this is disadvantageous in that what is ideal is that the maximum amount of membrane surface area be hydrophilic so that there is optimum liquid filtering efficiency.
Hence, there remains a need for a filter device, of the general type described, which provides optimum filtering efficiency while yet assuring against any accumulation of air or other gas on the inlet surface of the membrane.